Getting the Superfly render settings nailed down in Poser 13 seems very complex (and is different from Poser 12, so that old advice is not to be followed). You'll likely find it's about far more than the buckets (or as Blender Cycles users call them, 'tiles').  But basically, the manual https://www.posersoftware.com/documentation/13/HTML/Poser_Reference_Manual/Rendering/SuperFly/SuperFly_Render_Settings.htm says...

• Bucket Size: Smaller buckets use less memory. Larger buckets render the image faster.

When using GPU rendering, the Bucket Size setting will help to achieve optimal performance. Start with a bucket size between 100 and 300. Some scenes may benefit with larger settings.

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Learn the Secrets of Poser 11 and Line-art Filters.